(a) Field of the Invention
The present invention relates to a cathode for a cathode ray tube (CRT) and, more particularly, to a cathode which is used in a color CRT to display high-definition color picture images
(b) Description of the Related Art
Generally, in the CRTs, the cathode is designed to emit electrons for exciting phosphors coated on a faceplate panel. For the electron emission purpose, the cathode is provided with electron-emitting materials and, during operation, the electron-emitting materials are activated to produce the phosphor-exciting electrons.
FIG. 3 is a schematic cross-sectional view showing a conventional CRT cathode. As shown in FIG. 3, the cathode includes a base 31 having a cylindrical shape and an opening portion, and an electron-emitting material layer 32 coated on a closed top portion of the base 31 to emit thermal electrons. The base 31 contains nickel as its main component together with a small amount of reducing elements such as silicon, magnesium, etc. The electron-emitting material layer 32 is formed with alkali-earth metal oxides comprising barium, strontium and calcium.
The cathode further includes a heater 33 placed in the opening portion of the base 31 to heat it up to a predetermined temperature, and a control grid 34 positioned on a front portion of the electron-emitting material layer 32 to focus streams of the electrons emitted therefrom. A tungsten wire coated with alumina is commonly used as the heater 33.
In the meantime, the coating operation of the electron-emitting material layer 32 can be described as shown below.
First, a carbonate suspension containing alkali-earth metals such as barium, strontium, calcium, etc. is applied onto the top portion of the base 31 and heated by the heater 33 in a vacuum state. Then, the alkali-earth metal carbonates are changed into alkali-earth metal oxides. Subsequently, the alkali-earth metal oxides are again heated and aged so that some of the oxides are reduced through reacting with silicon, magnesium, etc. contained in the base 31 to be thereby endowed with a semiconducting property. In this way, the coating of the electron-emitting material layer 32 can be completed.
When the aforementioned electron-emitting material layer 32 is heated up to 800.about.900.degree. C. in normal conditions, it emits electrons with a current density of 0.5.about.0.8 A/cm.sup.2. However, since high-definition televisions have recently been developed, the need for a fine stream of electrons with a high current density is created and, hence, it is required to heighten the current density of the electrons up to 1.about.3 A/cm.sup.2. But, in case the conventional CRT cathode is employed for that purpose, the electron emission capacity of the cathode abruptly deteriorates so that it would be soon have to be discarded and replaced with a new one.
Therefore, the conventional CRT cathode has been currently improved and adapted to emit electrons with a current density of 1.about.3 A/cm.sup.2.
For example, Japanese Patent Laid-open No. Sho61-269828 discloses a technique of mixing the carbonate suspension of alkali-earth metals such as barium, strontium, calcium, etc. with an oxidizing scandium.
Furthermore, Japanese Patent Laid-open No. Hei2-33822 discloses a technique of attaching rare-earth metals or their compounds on the electron-emitting material layer. In the technique, the attachment is performed through applying the rare-earth metals or their compounds onto a cathode-side surface of the control grid and making the electrons emitted from the cathode to collide against the control grid. In this way, the electrons are scattered toward the electron-emitting material layer and attached thereon.
However, the experimental tests with respect to the CRT cathode manufactured in accordance with the technique disclosed in Japanese Patent Laid-open No. Sho61-269828 exhibited that the electron-emitting material layer 32 was slightly separated from the base 31, creating an unstable electron emission.
Furthermore, the experimental tests with respect to the CRT cathode manufactured in accordance with the technique disclosed in Japanese Patent Laid-open No. Hei2-33822 exhibited that the distance between the electron-emitting material layer 32 and the control grid 34 was extremely short compared to a diameter of the electron guide hole in the control grid 34. As a result, the scattered rare-earth metal oxides were not attached on the center portion of the electron-emitting material layer 32 opposite the electron guide hole. Thus, as a high current density is investigated with the cathode, the electron emission capacity is liable to deteriorate.